Recently, radio communications has come into widespread use remarkably owing to its convenience. As a result, there is an urgent demand for taking measures to deal with the shortage of use frequencies. As one of the techniques of using the use frequency effectively, there is a MIMO (Multiple-Input Multiple-Output) system for performing high-speed signal transmission using a plurality of antennas in a transmitter/receiver, which is under active studies. In the MIMO system, it is known that a higher capacity can be achieved by using a plurality of antennas in a transmitter/receiver, compared with the case where the transmitter/receiver has one antenna.
In the MIMO system, SDM (Space Division Multiplexing) transmission has been mostly studied, in which signals are sent individually from a plurality of transmission antennas, and each signal is extracted with signal processing on a receiving side. Hereinafter, a conventional technique will be described based on representative documents (for example, see Non-Patent Documents 1 and 2) related to the SDM transmission.
FIGS. 32 and 33 show a configuration of a transmitter/receiver performing the SDM transmission. In the SDM transmission, time-series signals are sent individually from respective antennas of a transmitter, and as shown in FIG. 33, a receiver receives the signals using beam formation corresponding to each transmission signal. A configuration of this signal processing will be described below. The description will be made assuming that the number of transmission antenna is N, the number of reception antennas is M, the channel gain from a transmission antenna n to a reception antenna m is hmn, and the propagation characteristics between the transmitter and the receiver is a matrix H=[hmn].
As shown in FIG. 32, at a terminal A1 of the transmitter, time-series transmission signals sn(p) (n=1, . . . , N) are transmitted from N transmission antennas 3. The transmission signals pass through a propagation path 5 to be received by M reception antennas 4. At a terminal B2 of the receiver, reception weight multiplying parts 131, 132, 133 multiply the reception signals with a weight vm to thereby perform signal combining.
Hereinafter, the above-mentioned series of processes will be shown using mathematical expressions. Assuming that a reception signal at the reception antenna 4 is xm(p), a reception vector x(p)=[x1(p), . . . , xm(p)]T (T is a transposition) is given by the following expression.x(p)=Σn=1Nhnsn(p)+z(p)
Herein, s1(p), . . . , sN(p) represents a transmission signal; hn=[h1n, . . . , hMn]T represents a propagation vector from the transmission antenna 3 to the M reception antennas 4; z (p)=[z1(p), . . . , zM(p)]T represents a noise and interference vector; and zm(p) represents a noise and interference component at the antenna 4.
Furthermore, the terminal B2 on the receiving side determines a weight vn=[vn1, . . . , vnM]T suitable for receiving the signal sn(p) from the transmission antenna 3. An output yn(p) after the signal combining is given by the following expression.yn(p)=vnTx(p)=Σn0=1N(vnThn0)sn0(p)+vnTz(p)
Although there are various methods for determining the reception weight vn, each reception weight vn is determined to get the transmission signal sn(p). For example, according to the weight determination based on a ZF (Zero Forcing) standard, the weight vn is determined so as to satisfy the following expressions.vnThn0=1 where n0=n. vnThn0=0 where n0 is other than n.  (Expression 1)
(Expression 1) shows the condition under which a desired signal sn(p) is received strongly, and the other signals sn0(p) (n0 is an integer other than n) are suppressed. Thus, only the desired signal can be received satisfactorily. Furthermore, by receiving a signal using different weights vn with respect to different n, a plurality of signals can be separated to be taken out, and hence, division multiplexing can be performed spatially. Herein, although a method for determining a weight based of the ZF standard has been described as an example, there is also a similar weight algorithm such as an MMSE synthesis method. The purpose of any weight algorithm is basically to suppress signals other than a desired one in the same way as in (Expression 1).
Thus, by suppressing signals other than a desired one among a plurality of signals at the terminal B2 on the receiving side, SDM (Space Division Multiplexing) can be realized. In the SDM transmission, a plurality of signals are transmitted simultaneously, so there is an advantage that high-speed signal transmission can be performed, compared with a conventional transmission system in which a transmitter/receiver uses a single antenna.
However, actually, although (Expression 1) can be realized in the case where the number N of multiplexed signals is M or less (N≦M), it cannot be realized in the case of N>M. In order to understand the contents thereof, more detailed description will be made. In (Expression 1), the vectors vn and hn0 can be respectively expressed as one vector on an M-dimensional space. Furthermore, vnThn0 being a vector inner product, and vnThno being 0 correspond to a state where vn and hn0 are orthogonal to each other on the M-dimensional space. Although one vector vn orthogonal to (M−1) independent vectors hn0 can be set on the M-dimensional space, it is impossible to set a vector vn orthogonal to M or more independent vectors hn0. Thus, it is theoretically impossible to satisfy the relationship vnThn0=0 with respect to M or more independent vectors hn0, and (Expression 1) does not hold for N>M.
Accordingly, in the case where the number N of multiplexed signals is larger than the number M of reception antennas, any weight vn used on the receiving side cannot suppress other signals sufficiently. Therefore, the quality of a reception signal degrades rapidly. In order to avoid this situation, there is required a method of performing space division multiplexing transmission smoothly in an environment where the number of transmission antennas is larger than the number of reception antennas. However, such solution measures have not been introduced as yet.    Non-Patent Document 1: A. V. Zelst, R. V. Nee, and G. A. Awater, “Space Division Multiplexing (SDM) for OFDM systems” IEEE Proc. of VTC 2000 Spring, pp. 1070 to 1074, 2000    Non-Patent Document 2: Kurosaki, Asai, Sugiyama, Umehira, “100 Mbit/s. SDM-COFDM over MIMO channel for broadband mobile communications” Technical Report, RCS 2001-135, October 2001